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Slightly OT (Maybe) - Are thermometer problems exponential?

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EggMcMic
EggMcMic Posts: 340
in EggHead Forum
Had a problem with my last couple cooks. Overdone/done way too fast...you get the idea. Had gotten a new Smokewear thermometer for Christmas, so I figured I'd switch them out. I took the opportunity to test both of them in boiling water. Sure enough, the new one read 212 (or close enough) and the old (original) one read about 180. So I guess I have two questions:
1. The thermometer had worked fine, based on cooks where I had a FlameBoss running side-by-side. Can they 'go bad'? Could some really cold weather have played a part?

2. When you have a thermometer that is 'off' by say 30 degrees (at 212 degrees), will it be off by 30 at 400? Or will it be off 60? Or more?

I really didn't think my cooks would have been affected as much as they were by 30 degrees which lead me to wondering if the difference was greater at higher temps. Thanks for any thoughts...
EggMcMcc
Central Illinois
First L BGE July 2016, RecTec, Traeger, Weber, Campchef
Second BGE, a MMX, February 2017
Third BGE, another large, May, 2017
Added another griddle (BassPro) December 2017
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  • fishlessman
    fishlessman Posts: 32,757
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    i have a short stem teltru, its off by a little over 10 percent.  heres the problem though, another gage i can set for 212, but it fails over 300, the slightest bend in the stem will do this. lastly, having two gages is always a problem =)
    fukahwee maine

    you can lead a fish to water but you can not make him drink it
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  • poster
    poster Posts: 1,172
    edited January 2018
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    I don't think the cold would have messed it up, ive had mine in -40 temps lots and its still fairly accurate

    Are you starting off 30 degrees over ambient temp? That should tell you if the error will get worse as you go.

    The degree increments are spaced evenly so normally if it was just an error of say the pointer was in the wrong spot then it shouldn't get worse as you go, but if the bi-metal part is not free or was over stretched then I could see the error increasing as you go. 

    Also normally grid and dome temps are around 30 degrees apart. When did your  side by side comparison, are they actually only an inch or 2 apart?

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  • lousubcap
    lousubcap Posts: 32,378
    edited January 2018
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    I definitely come down on the side of "believe your indication(s)."  So, I frequently single point (boiling water) check all the BGE thermos and my others (instant read thermos) especially prior to any low&slow cook.  Regarding your question, once out of cal then how it behaves at higher or lower temps is definitely a crap -shoot.  And who knows when they decide to fail.  BTW- my BGE tel-tru are solid as a rock and have been for several years.  (Now jinxed  ;) )
    If you can, calibrate your hosed-up Smokeware thermo so you can use it going forward.
    And like @fishlessman says, learn how your BGE works with one thermo and go from there.  I work with the dome thermo as every BGE has one but others have a different approach.  FWIW-
    Louisville; Rolling smoke in the neighbourhood. # 38 for the win.  Life is too short for light/lite beer!  Seems I'm livin in a transitional period.
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  • EggMcMic
    EggMcMic Posts: 340
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    Thanks for your thoughts @poster
    I agree with your thoughts on the pointer in the right spot/bi-metal issue. If it is a pointer issue then it is like setting a clock 10 minutes fast. I wasn't sure how the innerds worked and if an increasing error was possible.

    When I have done the side-by-side it would usually start about 30 degrees apart but as it went along (on a low-n-slow) they would even out to be the same after a few hours.
    EggMcMcc
    Central Illinois
    First L BGE July 2016, RecTec, Traeger, Weber, Campchef
    Second BGE, a MMX, February 2017
    Third BGE, another large, May, 2017
    Added another griddle (BassPro) December 2017
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  • EggMcMic
    EggMcMic Posts: 340
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    Sorry to have jinxed you @lousubcap , hope it still works. I have gotten pretty good at knowing the temperature of my egg, but both of these  cooks occurred in negative temperatures and I have relied more on the thermometers and less on me standing there freezing my a** off standing outside!
    EggMcMcc
    Central Illinois
    First L BGE July 2016, RecTec, Traeger, Weber, Campchef
    Second BGE, a MMX, February 2017
    Third BGE, another large, May, 2017
    Added another griddle (BassPro) December 2017
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  • berndcrisp
    berndcrisp Posts: 1,166
    edited January 2018
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    I don't think so, but.....

    Boil them ALL, calibrate to 212 or so. Measure temp with all of them when removing food and see if they match.

    Hood Stars, Wrist Crowns and Obsession Dobs!


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  • nolaegghead
    nolaegghead Posts: 42,102
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    They are not exponential
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  • DMW
    DMW Posts: 13,832
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    nolaegghead said:
    They are not exponential
    Are they logarithmic?
    They/Them
    Morgantown, PA

    XL BGE - S BGE - KJ Jr - HB Legacy - BS Pizza Oven - 30" Firepit - King Kooker Fryer -  PR72T - WSJ - BS 17" Griddle - XXL BGE  - BS SS36" Griddle - 2 Burner Gasser - Pellet Smoker
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  • nolaegghead
    nolaegghead Posts: 42,102
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    DMW said:
    nolaegghead said:
    They are not exponential
    Are they logarithmic?
    Wrong!
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  • nolaegghead
    nolaegghead Posts: 42,102
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    They are not exponential.

    Are they linear?
    Yes.   More or less.  But with limitations.

    http://physics.csustan.edu/Ian/Help/Temperature/Thermometers/BimetallicStripThermometer.htm

    Does a gas exhibit a linear relationship between temperature, pressure and volume?  Yes, but also with limitations.

    The "perfect" gas law PV=nRT works fine for predicting temperatures, pressures, etc.  But it's anything but perfect when pushed to, say, extremely low volumes and high pressures.  Then it gets complicated. 

    But for an oven thermometer, it's linear and if it's off 20F at 32F, it's probably 20F off 212F.   Unless the scale is wrong for the rate of bimetallic curl.  And they are always off some so you'll rarely be able to perfectly calibrate at freezing and boiling.  That's why it's better to calibrate at a temp that you need to measure than one way off the mark.  But it will still be linear.

    IMO temp accuracy isn't that important in roasting.  My LBGE therm has been broken for the past year and I really don't miss it.


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  • Carolina Q
    Carolina Q Posts: 14,831
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    nolaegghead said:

    IMO temp accuracy isn't that important in roasting.  My LBGE therm has been broken for the past year and I really don't miss it. 
    Like cooking over a campfire. No idea what the temp is, but the food is great!

    I hate it when I go to the kitchen for food and all I find are ingredients!                                                                                                                                                                                                                           

    Michael 
    Central Connecticut 

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  • Canugghead
    Canugghead Posts: 11,527
    edited January 2018
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    Not sure how Anova temp differs from dome temp ... my Anova is inaccurate and I keep it honest with a Thermapen, don't remember the exact numbers ... at 130-140'ish it reads about 1.5-2F too high, at higher temp the error creeps up to 2.5-3F, not big deal for egg cooking but makes significant difference in sous vide.
    canuckland
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  • nolaegghead
    nolaegghead Posts: 42,102
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    Canugghead said:
    Not sure how Anova temp differs from dome temp ... my Anova is inaccurate and I keep it honest with a Thermapen, don't remember the exact numbers ... at 130-140'ish it reads about 1.5-2F too high, at higher temp the error creeps up to 2.5-3F, not big deal for egg cooking but makes significant difference in sous vide.
    Thermocouples are not entirely linear in signal response, but close.  Thermistors are not linear at all, so a polynomial algorithm is used to determine temp.

    Image result
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  • JohnInCarolina
    JohnInCarolina Posts: 30,975
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    Nola tossing out all sorts of pearls today ;)
    "I've made a note never to piss you two off." - Stike
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  • nolaegghead
    nolaegghead Posts: 42,102
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    JohnInCarolina said:
    Nola tossing out all sorts of pearls today ;)
    I'm just making stuff up.   I have a random graph generator that I use for the pictures.
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  • nolaegghead
    nolaegghead Posts: 42,102
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    Image result for thermistor graph resistance temperature

    See?  See?
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  • SciAggie
    SciAggie Posts: 6,481
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    nolaegghead said:
    Image result for thermistor graph resistance temperature

    See?  See?
    I should do that. No one would probably know - or care... make up some big "sciencey" sounding words too.
    Coleman, Texas
    Large BGE & Mini Max for the wok. A few old camp Dutch ovens and a wood fired oven. LSG 24” cabinet offset smoker. There are a few paella pans and a Patagonia cross in the barn. A curing chamber for bacterial transformation of meats...
    "Bourbon slushies. Sure you can cook on the BGE without them, but why would you?"
                                                                                                                          YukonRon
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  • nolaegghead
    nolaegghead Posts: 42,102
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    Or say crazy nonsense like an exponential signal is the inverse of a logarithmic one.
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  • nolaegghead
    nolaegghead Posts: 42,102
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    No one cares unless you say something embellished with adjectives, adverbs and fancy understandable words. 

    Like, believe you me, this steak, is very very good, the best, will be cooked like the world has never seen before, perfectly, perfect, and I promise you, you will get tired of eating such good steak, better than anyone else's, mark my words!  Others?  Sad.
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  • Focker
    Focker Posts: 8,364
    edited January 2018
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    Folks seem to have issues with the Smokeware therms wandering. 

    I use, and prefer Tel Trus.  And I check them annually.  Have yet to adjust after the initial dial in.

    For my style of cooking and preheat on the eggs, an accurate dome temp helps when I pick a target to hold.  Kettles, WSMs, not so much.

    There is a difference in roasting/browning, baking, braising from 350, to 450, to 550.
    Brandon
    Quad Cities
    "If yer gonna denigrate, familiarity with the subject is helpful."

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  • lousubcap
    lousubcap Posts: 32,378
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    @JohnInCarolina and @nolaegghead - you definitely appreciate using the graph scales to tell/spin the story.  Unfortunately so do many of the media and the commentators have no idea what they are saying-just reading the script.  With enough numbers you can reach any conclusion you want.  Nothing new and won't be.  
    At least with Q the taste test cuts thru the photogenic qualities of the camera super-phone pics. ;)   Have a great evening.
    Louisville; Rolling smoke in the neighbourhood. # 38 for the win.  Life is too short for light/lite beer!  Seems I'm livin in a transitional period.
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  • nolaegghead
    nolaegghead Posts: 42,102
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    lousubcap said:
    @JohnInCarolina and @nolaegghead - you definitely appreciate using the graph scales to tell/spin the story.  Unfortunately so do many of the media and the commentators have no idea what they are saying-just reading the script.  With enough numbers you can reach any conclusion you want.  Nothing new and won't be.  
    At least with Q the taste test cuts thru the photogenic qualities of the camera super-phone pics. ;)   Have a great evening.
    @lousubcap I was kidding about making up graphs... =)

    I try to be as honest as possible when using graphs to display information and trends. 

    Disclaimer:  I can't help those that are too ****** stupid to understand them.
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